Optical blood monitors have been widely used in connection with conventional intermittent hemodialysis systems in which extracorporeal blood flow rate is typically in the range of 200 to 1500 ml/min. Optical blood monitors measure the patient's hematocrit value and/or oxygen saturation level in real time. It is well known that the change in blood volume is inversely proportional to the hematocrit value and also that the hemoglobin level can be estimated from the hematocrit value.
Applicants' assignee sells the Crit-Line® optical blood monitoring system. The Crit-Line® blood monitoring system uses optical techniques to non-invasively measure in real time the hematocrit level (HCT) of blood flowing through extracorporeal tubing, normally ¼″ tubing associated with a dialysis machine. In the Crit-Line® system, a sterile, single-use blood chamber is attached inline to the extracorporeal tubing usually located on the arterial side of the dialyzer. The blood chamber provides a viewing point for optical sensors. Multiple wavelengths of visible and infrared light are directed through the blood chamber and the patient's blood flowing through the blood chamber, and a photodetector measures the resulting intensity of light at each wavelength. The preferred wavelengths are about 810 nm (e.g. 829 nm), which is substantially isobestic for red blood cells, and about 1300 nm, which is substantially isobestic for water.
A ratiometric technique is implemented in the Crit-Line® controller to calculate the patient's hematocrit value in real time. The ratiometric technique is substantially disclosed in U.S. Pat. No. 5,372,136 entitled “System and Method for Non-invasive Hematocrit Monitoring”, which issued on Dec. 13, 1999 and is also assigned to the assignee of the present application. The hematocrit value (HCT) is defined as the percentage determined by dividing the volume of red blood cells in a given whole blood sample by the overall volume of the blood sample. A screen on the Crit-Line® controller typically displays a graph showing the percentage change in blood volume during a dialysis treatment session. The Crit-Line® system can also measure, optically, the oxygen saturation level in the blood flowing through the dialysis system as related to the patient's condition.
Typically, the photoemitters and detector are located in a clip assembly that clips over the viewing area for the blood chamber. An electrical cable connects the head of the clip to the controller, which is located near the patient and near the hemodialysis machine. The clip assemblies are calibrated at the factory, and it may be necessary to recalibrate the clip assemblies from time to time, as described in co-pending patent application Ser. No. 12/265,392, filed Nov. 5, 2008, entitled “Measuring Hematocrit and Estimating Hemoglobin Values with a Non-Invasive, Optical Blood Monitoring System”, incorporated by reference herein. The calibration of the clip assembly depends at least in part on the configuration of the blood chamber viewing area.
The Crit-Line® blood chamber that is in use today includes an inlet port and channel which lead to an internal blood flow cavity and an outlet channel and port which lead away from the cavity for the blood to exit the blood chamber. Connectors commonly called luer locks are provided to connect the blood chamber to extracorporeal tubing or a dialysis filter as required. The internal blood flow cavity is defined by two circular flat walls that are parallel to one another and set apart at a predetermined fixed distance. It is important that the thickness of the lenses in the viewing area as well as the predetermined fixed distance between the lenses be tightly controlled during manufacturing inasmuch as calibration of the sensing assembly and controller depends on these dimensions. The blood chamber comprises a molded, clear polycarbonate chamber body which includes an integral viewing lens on one side of the blood flow cavity. A lens body including the clear polycarbonate lens is sonically welded to the chamber body and provides a viewing lens on the other side of the blood flow cavity. A moat providing a ring of thicker blood depth surrounds the circular internal blood flow cavity. One of the primary purposes of the moat is to protect visible and infrared light from ducting through the body of the blood chamber directly to the photodetector(s); rather than passing through a direct path from the photoemitter to the detector through the blood flow. The blood chamber also includes two turbulence posts located in the vicinity of the location where the inlet channel opens into the circular blood flow cavity. The purpose of the turbulence post is to establish standing eddy currents within the circular blood flow cavity in order to provide a reliable, homogeneous blood flow through the viewing area. When designing the blood flow path through the blood chamber, it is important to eliminate or minimize the propensity for blood to clot in the blood chamber, and also eliminate the possibility of hemolysis as the blood flows into and through the blood chamber. The current Crit-Line® blood chamber, as described, is designed to accommodate flow rates of about 100 ml/min. to 1500 ml/min. through ¼″ extracorporeal tubing. Under these conditions, the current design has been found to provide safe, consistent, homogeneous flow through the blood chamber (without clotting and hemolysis problems) which in turn leads to accurate hematocrit and blood volume data.
As mentioned, the photoemitters and detector(s) are contained in a clip assembly that clips over the blood chamber on either side of the chamber walls forming the internal blood cavity and viewing lenses. The outer surfaces of the blood chamber include detents that provide access for the faces of the sensor assembly to rest against the outer flat surface of the respective viewing lens or be spaced a fixed distance from the viewing lens as the case may be. The shape of the detents is configured such that the clip assembly can fit onto the blood chamber only in a pre-selected geometric orientation in which the photoemitters and detector(s) are aligned in known positions.
It is an object of the invention to provide an optical blood chamber suitable for use in low flow applications, e.g. 10 ml/min. to 500 ml/min. flowing through ⅛″ extracorporeal tubing, such as low flow kidney dialysis or aquaphoresis treatments such as for preventing congestive heart failure. It is a further object of the invention to develop such a low flow blood chamber that is compatible for use with the existing sensor clip assemblies and control systems.